Fail-safe operating method for a decentralized power generation plant

ABSTRACT

A fail-safe operating method for a decentralized power generation plant DG includes determining a leakage capacitance of a generator of the DG before connecting the DG. The method also includes comparing the determined leakage capacitance with a predetermined first limit value, and connecting the DG to a grid only if the determined leakage capacitance is smaller than the predetermined first limit value. A decentralized power generation plant is configured to perform the method.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/893,276, filed on Jun. 4, 2020, which is a continuation ofInternational Patent Application number PCT/EP2018/083229, filed on Nov.30, 2018, which claims priority to German Patent Application number 102017 129 083.4, filed on Dec. 6, 2017, and is hereby incorporated byreference in its entirety.

FIELD

The disclosure relates to an operating method for a decentralized powergeneration plant with the aim of avoiding unnecessary shutdown of thedecentralized power generation plant due to an excessive residualcurrent, and a decentralized power generation plant configured toperform this method.

BACKGROUND

Decentralized power Generation plants (hereinafter referred to as DGs),in particular photovoltaic plants, are often equipped with a residualcurrent circuit breaker that disconnects a connection of the DG to agrid if a difference between the sum of the currents flowing out of theDG and the sum of the currents flowing into the DG exceeds a limitvalue. Such a limit value excess of the residual current may be causedby inadequate insulation of the live components from the earth potentialand may be due to accidental contact with a live line by a person, forexample. Often it is required by standards that after the system hasbeen switched off, the system may only be switched on after it has beenchecked by maintenance personnel, so that the system cannot feed energyinto the grid for a long period of time after being switched off, evenif another cause has caused the system to be switched off and a safereconnection to the grid would be possible.

At the same time, especially in the case of transformerless inverters aspart of a DG, the connecting lines to a generator of the DG show avariation of the line potential with respect to earth due to the design,so that a grid-frequency leakage current component arises which dependson the level of the leakage capacity of the generator with respect toearth. At a high value of the leakage capacitance, the grid frequencyleakage current component can assume an amplitude which leads to anearly tripping of the residual current circuit breaker. The value of theleakage capacitance depends on the size of the generator as well as onthe way it is installed and is also strongly dependent on the weatherconditions. Especially in wet conditions, the leakage capacitance of aDG rises sharply and can cause the residual current circuit breaker totrip regularly under these weather conditions due to high leakagecurrents, which are incorrectly detected as impermissible residualcurrents, and thus trip the residual current circuit breakerunnecessarily.

Therefore, various approaches exist to avoid the described prematuretripping of the residual current circuit breaker. For example, GB 2 258095 A proposes to divide the residual current into a direct currentcomponent, a first line-frequency current component which is in phasewith a frequency of the grid, and a second line-frequency leakagecurrent component which is 90° out of phase with a frequency of thegrid. Different threshold values are assigned to the differentcomponents. If these values are exceeded, the DG disconnects from thegrid.

In contrast, EP 0 878 850 A2 proposes to design an installation only insuch a way that the leakage capacity of the generator of a DG does notexceed a predetermined value calculated from a trip threshold of aresidual current circuit breaker. This limits the size of the generatorand thus the system output.

Another known solution is to feed a grid frequency compensation currentinto one of the grid supply lines monitored by a residual current sensorof the residual current circuit breaker, which compensates the gridfrequency leakage current component, or to impress the compensationcurrent into an additional line or winding passing through the residualcurrent sensor. The compensation current must be adapted to therespective operating state of the system by means of a control device.

In the light of the known state of the art, there is a need for acost-effective and reliable residual current monitoring system that isrobust against weather-induced false tripping of the residual currentcircuit breaker and does not unnecessarily restrict the design of theDG.

SUMMARY

A fail-safe operating method for a decentralized power generation plantDG comprises determining a leakage capacitance of a generator of the DGbefore connecting the DG to a grid, comparing the determined leakagecapacitance with a predetermined first limit value, and connecting theDG to the grid only if the determined leakage capacitance is smallerthan the predetermined first limit value.

Due to the conditional connection to the grid, a line-frequency leakagecurrent component caused by the leakage capacitance remains limited insuch a way that it remains small compared to the threshold value atwhich a residual current circuit breaker trips and disconnects the DGfrom the grid. The condition ensures that in operating situations inwhich the leakage current component represents a significant portion ofthe tripping threshold—thus significantly reducing the shutdown marginso that the probability is high that the residual current circuitbreaker will trip unnecessarily—the DG remains disconnected from thegrid and does not switch on until the condition is fulfilled again. Inpractice, it has been found that the loss of yield caused by the DGremaining temporarily disconnected from the grid when the condition isnot met is small compared to the loss of yield caused by an unnecessarytrip of the residual current circuit breaker due to a system shutdownuntil the DG is manually tested.

At the same time, it is not necessary to elaborately compensate orotherwise take into account this leakage current component whendetermining a residual current. The residual current circuit breaker cantherefore be designed in a less complex way. A premature and unnecessarytriggering of the residual current circuit breaker is neverthelessprevented by using the condition according to the disclosure.

After connecting the DG to the grid, the determination of a capacitiveleakage current component of the residual current of the DG into thegrid can be determined continuously or repeatedly. If the determinedcapacitive leakage current component exceeds a second limit value, theDG can be temporarily disconnected from the grid again. A renewedconnection of the DG at a later time can be carried out in particular byusing the inventive operating method described above.

As an alternative to disconnecting the DG from the grid, however,exceeding the second limit value by the capacitive leakage currentcomponent can instead cause the generator voltage of the DG generator tobe temporarily reduced. Although this does not necessarily lead to areduction of the capacitive leakage current, it does lead to a reductionof an additional current flowing via an insulation resistance, so thatthe probability of unnecessary tripping of the residual current circuitbreaker is reduced by this measure as well. As a result, the DG cancontinue to produce energy and the capacitive leakage current componentcan be continuously monitored during operation of the DG, so that themeasure of reduced generator voltage can be immediately cancelled againif the voltage falls below the second limit value. The loss of yield dueto the required countermeasure can thus be kept low. However, if thecapacitive leakage current continues to rise, the DG can of course bedisconnected from the grid before the residual current circuit breakertrips.

In an advantageous implementation of the method according to thedisclosure, the second limit value can be chosen to be less than orequal to half of a nominal tripping threshold of the residual currentcircuit breaker of the DG. In this way, it can be ensured that theresidual current circuit breaker does not trip exclusively on the basisof the capacitive leakage current component during operation of the DGwithout an insulation residual being present, even if the residualcurrent circuit breaker has an individual tripping threshold that ispermissibly below the nominal tripping threshold.

In one embodiment, the leakage capacitance is determined together withthe insulation resistance of the generator. The DG has an insulationmonitoring system for this purpose. For example, a voltage amplitudewith a measuring frequency, which can also deviate from the gridfrequency, is applied between a connection of the generator and areference potential, for example, an earth potential. By measuring thecurrent caused by this, the insulation monitoring can jointly determineboth, the insulation resistance and the leakage capacitance.

To determine the leakage capacitance, at least two voltage values can bespecified on connecting lines of the generator of the DG by means of aninverter bridge of an inverter of the DG. The currents or currenttransients caused by the two voltage values can be used to determine theleakage capacitance. The specification of a sinusoidal voltage curveusing the inverter bridge in one example.

In an advantageous variant of joint determination of leakage capacitanceand insulation resistance, the neutral conductor connection of theinverter is connected to the neutral conductor of the grid, while thephase connections remain separate. Subsequently, a voltage of apredetermined amplitude and frequency can be applied to the generatorconnections via a suitable control of the inverter bridge. The resultantcurrent can be measured with already present current sensors andevaluated to determine the leakage capacitance and insulationresistance.

This method can also be used to test the tripping threshold of theresidual current circuit breaker by increasing the amplitude of thevoltage specified with the inverter bridge stepwise or continuouslyuntil the residual current circuit breaker trips, for example, using thegrid frequency as the measuring frequency. The amplitude of the currentcaused by the voltage when the residual current circuit breaker istripped determines the tripping threshold of the residual currentcircuit breaker. Provided that it is ensured that the DG has asufficiently high insulation resistance when carrying out this method,and thus the resultant current contains only the capacitive leakagecurrent component, the tripping threshold corresponds to the currentamplitude when the residual current circuit is tripped.

In one embodiment, preference is given to the second limit value beingdetermined as a function of the tripping threshold thus determined, inparticular by reducing the second limit value by a predetermined amountor a predetermined percentage compared with the determined trippingthreshold.

In a further advantageous variant of the method according to thedisclosure, an unnecessary tripping of the residual current circuitbreaker during operation of the DG is reacted to by an adjustment of thefirst limit value by lowering the first limit value depending on adifference between the leakage capacitance determined during the lastconnection of the DG to the grid and the previous first limit value. Inone embodiment, the first limit value can be lowered by the entiredifference or by a specified percentage of this difference. In this way,it is possible to react to an unnecessary trip of the residual currentcircuit breaker due to an inappropriately high first limit value byautomatically reducing the first limit value to a sufficiently low valueto prevent unnecessary trip of the residual current circuit breaker.

To further improve the method, current weather data and/or weatherforecast data can be taken into account. For example, the first limitvalue can be selected depending on the weather and/or weather forecastdata. In this way, the method can take into account a weather-relatedchange in the leakage capacitance determined at the time of switching onduring the course of the expected operating time of the DG and, forexample, refrain from switching on the DG if the leakage capacitance hasalready almost reached the first limit value and a further increase inthe leakage capacitance is foreseeable due to the onset ofprecipitation.

As an alternative to not connecting the DG to the grid, the currentweather data and/or weather forecast data can also be taken into accountby the DG first connecting to the grid, but already setting a time atwhich it will disconnect from the grid again. This point in timecorresponds to the time of a forecast weather change that is expected toresult in a critical increase in the leakage capacitance. This methodcan be chosen, for example, if the leakage capacitance determined whenthe device is connected is less than a specified distance value belowthe first limit value.

In another embodiment, the disclosure includes a DG set up to performthe operating method described above. By using this method, it ispossible to also use such residual current sensors within the residualcurrent circuit breaker in which no compensation of a capacitive leakagecurrent component takes place. Nevertheless, it is possible to reliablyprevent tripping of the residual current circuit breaker due to anexcessive leakage current component.

Advantageously, the DG includes an insulation monitoring system that isadapted for the joint determination of an insulation resistance and theleakage capacitance of a generator of the DG.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the disclosure is represented by means of figures, ofwhich

FIG. 1 shows an embodiment of a DG according to the disclosure, and

FIG. 2 shows a flow chart of an operating method according to thedisclosure.

DETAILED DESCRIPTION

FIG. 1 shows a DG 1, which is designed as a photovoltaic system with aninverter 10, wherein the inverter 10 converts the direct current of agenerator 11 into a grid-compliant alternating current for feeding intoa grid 18 via grid connection lines by means of an inverter bridge. Agrid relay 12 is arranged in the grid connection lines of the DG 1,which is controlled by a residual current circuit breaker 14. Theresidual current circuit breaker 14 is connected to a residual currentsensor 13 which detects a residual current on the grid connection linesand compares it with a threshold value. If the threshold value isexceeded, the grid relay 12 is opened to disconnect the inverter 10 fromthe grid 18.

Furthermore, the DG 1 has an insulation monitoring circuit, sensor ordevice 15 on the connection lines of the inverter 10 to the generator11, which is configured to determine the value of an insulationresistance 16 and the value of a leakage capacitance 17 of the generator11 with respect to an earth potential before the DG 1 is connected tothe grid 18 and/or during operation of the DG 1. The insulationresistance and the leakage capacitance are not real components, butshould represent the electrical properties of the DG as an equivalentcircuit diagram and can also be connected at any other point of thegenerator 11.

Depending on the values of the insulation resistance 16 and the leakagecapacitance 17 determined in this way, the insulation monitoring device15 transmits a signal to the residual current circuit breaker 14, whichis used to control the grid relay 12. In one embodiment, it is providedthat if the insulation resistance 16 falls below a minimum insulationresistance or if the leakage capacitance 17 exceeds a maximum leakagecapacitance, the grid relay 12 is not closed or is opened if it isclosed.

FIG. 2 shows an inventive operating method for a DG such as the DG 1shown in FIG. 1 In a first act 20, the leakage capacitance (C_(L)) of agenerator in the DG is determined at the beginning of the method beforeconnecting the DG. In a second act 21, the value of the leakagecapacitance (C_(L)) determined in this way is compared with apredetermined first limit value (C_(TH)) and, if it is exceeded (YES at21), the DG branches back to the beginning of the method and thedetermination of the leakage capacitance is repeated at a later point intime without the DG being connected to the grid.

In a third act 22, the DG is connected to the grid if the determinedleakage capacitance is less than or equal to the specified limit value(NO at 21).

The third act 22 can be followed by further procedural acts after the DGhas been commissioned, in which the leakage capacitance continues to bemonitored. For example, after connecting the DG to the grid, acapacitive leakage current component of a residual current of the DG canbe determined continuously or repeatedly and, if a second limit value isexceeded by this leakage current component, the system can bedisconnected from the grid or alternatively a generator voltage of thegenerator can be reduced.

Furthermore, cases of tripping of the residual current circuit breakerof the DG can be evaluated to the effect that the first or second limitvalue is suitably adjusted if unnecessary shutdowns continue to occurdue to an excessively high capacitive leakage current component.

Alternatively or additionally, the second limit value can be determinedsuitably with the following acts: first, a capacitive leakage currentcomponent is determined at which the residual current circuit breakertrips. The value of this component is determined in one embodiment underthe condition that no other leakage current components are present. Thenthe second limit value is determined as a value reduced by apredetermined amount or by a predetermined percentage compared to thetripping capacitive leakage current component.

Furthermore, the first limit value can be selected depending on currentweather data and/or weather forecast data. In this way it can be takeninto account to what extent the leakage capacitance can vary due toweather conditions compared to a leakage capacitance determined at thetime of connection to the grid, in order to be able to estimate thepossibility of increasing the leakage capacitance to a value criticalfor residual current disconnection in the course of the operating period(usually the remaining day). In this way, it can be decided to waive theyield of the installation on the day in question, in order to avoid theeffort of manually resetting the residual current circuit breaker. It isalso conceivable to provide for suitable measures to avoid a residualcurrent related shutdown of the DG, for example its temporarydisconnection from the grid, within a period determined by the weatherforecast.

1-20. (canceled)
 21. A decentralized power generation plant (DG), comprising: an insulation monitoring circuit configured to determine a value of a leakage capacitance of a generator when the generator is coupled to input terminals of the DG, and a residual current circuit breaker coupled to the insulation monitoring circuit and configured to control a grid relay of the DG based on the determined leakage capacitance to ensure that the DG remains disconnected from a grid when the determined leakage capacitance satisfies a predetermined criteria.
 22. The DG according to claim 21, wherein the residual current circuit breaker is configured to control the grid relay by ensuring that the grid relay is open when the determined leakage capacitance satisfies the predetermined criteria.
 23. The DG according to claim 22, wherein the predetermined criteria being satisfied comprises the determined leakage capacitance exceeding a predetermined threshold.
 24. The DG according to claim 21, further comprising a residual current sensor configured to determine a residual current on connection lines that couple the DG to the gird.
 25. The DG according to claim 24, wherein the residual current sensor is uncompensated with respect to a leakage capacitance sensor of the insulation monitoring circuit.
 26. The DG according to claim 24, wherein the residual current circuit breaker is configured to open the grid relay if the residual current determined by the residual current sensor exceeds a maximum residual current value.
 27. The DG according to claim 21, wherein the insulation monitoring circuit is further configured to jointly determine an insulation resistance and the leakage capacitance of the generator.
 28. The DG according to claim 21, further comprising a data interface configured to receive weather data or weather forecast data, and wherein the decentralized power generation plant is configured to select the predetermined criteria based on received weather data or weather forecast data.
 29. A method of operating a decentralized power generation plant (DG), comprising: prior to connecting the DG to a grid, applying at least two voltage values to connecting lines of a generator of the DG, determining a leakage capacitance of the generator of the DG based on currents or current transients caused by the application of the at least two voltage values, and selectively connecting the DG to the grid based on the determined leakage capacitance.
 30. The method according to claim 29, wherein the at least two voltages are applied by an inverter bridge of the DG.
 31. The method according to claim 29, wherein selectively connecting the DG to the grid comprises connecting the DG to the grid only when the determined leakage capacitance is smaller than a predetermined first limit value.
 32. The method according to claim 29, wherein determining the leakage capacitance is carried out together with determining an insulation resistance of the generator.
 33. The method according to claim 31, wherein after connecting the DG to the grid, the method further comprises: determining a capacitive leakage current component of a residual current of the DG continuously or repeatedly, and disconnecting the DG from the grid when the determined capacitive leakage current component exceeds a second limit value.
 34. The method according to claim 33, wherein the second limit value is selected to be less than or equal to half of a nominal tripping threshold of a residual current circuit breaker of the DG.
 35. The method according to claim 33, wherein after connecting the DG to the grid, the method further comprises: determining a capacitive leakage current component of a residual current of the DG continuously or repeatedly, and reducing a generator voltage of the generator when the capacitive leakage current component exceeds the second limit value.
 36. The method according to claim 35, wherein the second limit value is determined by: determining a capacitive leakage current component at which the residual current circuit breaker of the DG trips, and determining the second limit value to be a value reduced by a predetermined amount or by a predetermined percentage compared to the capacitive leakage current component at which the residual current circuit breaker of the DG trips.
 37. The method according to claim 35, wherein the second limit value is selected to be less than or equal to half of a nominal tripping threshold of a residual current circuit breaker of the DG.
 38. The method according to claim 31, further comprising: lowering the first limit value in response to a trip event of a residual current circuit breaker of the DG as a function of a difference between leakage capacitance determined prior to the trip event and an initial first limit value.
 39. The method according to claim 31, wherein the first limit value is selected based on current weather data and/or weather forecast data.
 40. The method according to claim 31, wherein in a case of connection to the grid and when the determined leakage capacitance is below the first limit value by less than a predetermined amount, selecting a time at which the DG is disconnected from the grid again based on current weather data and/or weather forecast data. 